1. Field of the Invention
The present invention relates to technology that improves communication quality in a communication system that widens a band per mobile communication device by combining a plurality of antennas.
2. Description of the Related Art
A so-called “far-near problem” occurs in mobile radio communication systems. This problem occurs when all terminals output radio waves with the same transmit power regardless of the distance of the terminals from a base station, and as a result the radio waves of a terminal that is near to the base station become too strong and it is not possible to separate the radio waves from a terminal that is far from the base station.
To solve the far-near problem and also to save on transmit power, TPC (Transmit Power Control) technology is used in which a base station controls the transmit power of an antenna of each terminal based on a power of the reception signals that is received from each terminal.
As disclosed in “Texas Instruments Source, “3GPP TSG RAN WG1 51bis Sevilla”, [online], 14 Jan. 2008, [search conducted on 26 May, 2008], Internet URL: http://www.quintillion.co.jp/3GPP/TSG_RAN/TSG_RAN2008/TSG_RAN_WG1_RL1—1.html” (hereunder, referred to as “Non-patent Document 1”), according to the TPC technology each terminal outputs a signal for SIR measurement to cause a base station to measure a signal to interference power ratio (SIR). The base station receives the signals for SIR measurement and calculates SIRs that correspond to the respective antennas. Subsequently, the base station determines differences between a predetermined SIR target value and the calculated SIRs, and generates TPC commands that indicate a transmit power control amount for each antenna so as to reduce the differences.
As described in “W-CDMA Mobile Communication System”, edited by Keiji Tachikawa, Maruzen Co., Ltd, Publishing Division, 25 Jun. 2006, pp. 53-55, pp. 127-129” (hereunder, referred to as “Non-patent Document 2”), for example, this TPC command is notified at intervals of one millisecond or the like to the terminal from the base station through a field provided for notifying UL (Up Link) information in a PDCCH (Physical Downlink Control Channel).
In an MIMO (Multiple Input Multiple Output) system in which a terminal is provided with a plurality of antennas, as disclosed in Japanese Patent Laid-Open No. 2005-57497 (hereunder, referred to as “Patent Document 1”), a base station determines a total value of assessment values based on an SIR of each antenna for each combination of the transmit powers of each antenna. By controlling the transmit power of each antenna so as to minimize the total value, the base station can improve the communication quality. Further, an MIMO system disclosed in International Patent Application No. WO 2005/004376 pamphlet (hereunder, referred to as “Patent Document 2”) uses a base station to determine the existence/non-existence of errors in a data stream for each antenna. When an error exists, the MIMO system retransmits the relevant data to the terminal to thereby improve the communication quality.
However, even in the MIMO systems disclosed in Patent Documents 1 and 2, there are cases in which the communication quality declines.
More specifically, when controlling a plurality of antennas, the number of TPC commands to be transmitted within a predetermined period increases in proportion to the number of antennas. Therefore, in order to be able to transmit a TPC command to all antennas, the base station transmits a radio frame including a TPC command for each antenna by time division.
However, when the number of antennas is too great with respect to the communication speed, even when transmission is performed using time division it is not possible to transmit a TPC command to all antennas within a TPC update period (for example, 2 milliseconds).
For example, in a case in which a TPC command is updated every two milliseconds and a TPC command is transmitted every one millisecond to three antennas, respectively, the TPC command can be transmitted to only two of the three antennas within the two milliseconds, and transmit power control of the remaining antenna is delayed.
Thus, because control of the transmit power is delayed on the transmitting side, it is difficult to separate the radio waves for each antenna on the receiving side, and there is the problem that the communication quality of the transmission channels corresponding to the antennas declines.